Evaluation of the Cary Absolute Specular Reflectance accessory for the measurement of optical constants of thin films

Significance of the topic

Accurate determination of optical constants of thin absorbing films is essential for the design and quality control of optical coatings, semiconductor devices, photovoltaics and other optoelectronic applications. Knowledge of the refractive index (n) and extinction coefficient (k) enables calculation of dielectric functions, absorption coefficients and guides material selection and film processing parameters.

Objectives and study overview

This study presents a preliminary evaluation of the newly released Cary Absolute Specular Reflectance Accessory (SRA) when paired with a Cary 2400 spectrophotometer. The goals are to assess instrument performance across the UV-Vis-NIR spectrum, validate alignment stability, and determine the feasibility of measuring front and rear film reflectance to derive thin‐film optical constants.

Methodology and instrumentation

Measurements were carried out by recording specular reflectance from the front (R) and rear (R1) faces of a substrate‐film system along with transmittance (T) over 200 nm to 3000 nm. A polished <100> silicon wafer served as a reflectance standard. Film samples comprised glass slides coated with aluminum films deposited in the laboratory. Two key alignment tests were performed:

• Rotation sensitivity: the sample was tilted about a vertical axis in 0.1° increments to assess reflectance variation.
• Displacement sensitivity: the reflecting surface was moved up to 1 mm away from the sample plane to evaluate signal changes.

Instrumentation

• Cary 2400 UV-Vis-NIR spectrophotometer (185 nm to 3152 nm range)
• Cary Absolute Specular Reflectance Accessory (SRA) with aluminum-coated mirrors
• Photomultiplier and PbS detectors for visible and near-infrared regions

Results and discussion

The cumulative reflectivity loss of the three-mirror aluminum SRA design was mapped, showing a notable dip around 820 nm due to mirror absorption and detector changeover effects at 800 nm. Reflectance measurements on the silicon standard matched literature values from spectroscopic ellipsometry within experimental uncertainty. Alignment tests demonstrated that rotations up to ±0.2° had negligible impact on measured reflectance and displacements up to 1 mm led to less than 1.5% variation, indicating robust performance under realistic sample mounting tolerances.

Benefits and practical applications

The Cary SRA offers:

• High signal‐to‐noise ratio across UV-Vis-NIR, even near detector transition zones.
• Symmetric front and rear beam paths for consistent baseline corrections.
• Front-loading sample holder that accommodates smaller specimens and simplifies handling.

Future trends and applications

Further advancements may include integration of dielectric mirror coatings to extend high reflectivity beyond 820 nm, automated alignment routines for unattended operation, and in-situ monitoring capabilities for real-time thin-film growth studies. Expansion into mid-infrared spectroscopy and coupling with ellipsometric analysis could broaden application scope in advanced material research.

Conclusion

Preliminary evaluation indicates that the Cary Absolute Specular Reflectance Accessory delivers reliable, high-quality reflectance data for thin-film optical constant determinations. Its robust alignment tolerance, broad spectral range and user-friendly sample loading make it suitable for routine analytical work in research and industrial laboratories.

References

1. McPhedran, R. C.; Ross, C.; Watson, L. R. Unambiguous determination of optical constants of absorbing films by reflectance and transmission measurements. Applied Optics 23, 1197–1202 (1984).
2. Gourlet, D. L. Spectrophotometric measurements of filters, laser reflections and optical materials. Instruments At Work UV-23, 3–10 (1982).
3. Aspnes, D. E.; Studna, A. A. Dielectric functions and optical parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs and InSb from 1.5 to 6.0 eV. Physical Review B 27, 985–1009 (1983).